Novel Hybrid Energy Conversion and Storage Cell with Photovoltaic and Supercapacitor Effects in Ionic Liquid Electrolyte

A single hybrid energy conversion and storage (HECS) cell of alpha-cobalt hydroxide (α-Co(OH)2) in ionic liquid was fabricated and operated under light illumination. The α-Co(OH)2, which is unstable in an aqueous electrolyte (i.e. KOH), is surprisingly stable in 1-butyl-1-methyl-pyrrolidinium dicyanamide ionic liquid. The as-fabricated HECS cell provides 100% coulombic efficiency and 99.99% capacity retention over 2000 cycles. Under a photo-charging condition, the dicyanamide anion of ionic liquid can react with a generated α-Co(OH)2+ hole at the positive electrode since the HOMO energy level of the anion is close to the valence band of α-Co(OH)2. The excited photoelectron will transfer to the current collector and move to the negative electrode. At the negative electrode, the 1-butyl-1-methyl-pyrrolidinium cations of ionic liquid do electrostatically adsorb on the surface and intercalate into the interlayer of active material stabilizing the whole cell. The HECS cell having both energy conversion (photovoltaic effect) and energy storage (supercapacitor) properties may be an ideal device for future renewable energy.


Crystallinity of the as-electrodeposited Co(OH)2
Figure S1. GIXRD pattern (a), the diffraction rings of as-electrodeposited Co(OH)2 (b) and XRD pattern of the powder of Co(OH)2 before (black line) and after (red line) immersed in 6 M KOH (c).
An GIXRD pattern of as-electrodeposited Co(OH)2 film is shown in Fig.S1a. The 101 and 110 planes display higher intensity than other peaks. The diffraction ring of the as-electrodeposited Co(OH)2 from TEM technique is shown in Fig.S1b. Two diffraction rings can be clearly observed and their SAED analysis can be ascribed to 101 and 110 planes. The XRD patterns of the powder of the aselectrodeposited Co(OH)2 film before (black line) and after (red line) immersed in 6 M KOH are shown in Figure S1c. The XRD peaks of -Co(OH)2 and -Co(OH)2 are clearly different. The powder of the as-electrodeposited Co(OH)2 film before immersed in 6 M KOH (black line) is mainly -Co(OH)2, whilst after immersed in 6 M KOH (red line) -Co(OH)2 is mainly found. From the XRD data, the dspacing of -Co(OH)2 calculated from 003 plane (2 = 10.1  ) is ca. 8.8 Å. Whist, the d-spacing of -Co(OH)2 calculated from 001 plane (2 = 19.9  ) is ca. 4.5 Å. Figure S2. EDX patterns and the elemental mapping analysis (Co, O, C, and N) of the aselectrodeposited Co(OH)2 and the as-tested Co(OH)2 in ionic liquid electrolyte.

Energy-dispersive X-ray spectroscopy (EDX)
The Co(OH)2 electrode before and after tested was investigated by EDX technique with the elemental mapping of Co, O, C and N as shown in Figure S2. The EDX displays the Co:O ratio of 1:3. To study the effect of ionic liquid on the performance of the symmetric Co(OH)2 supercapacitor, the properties of ionic liquid were investigated with Raman and FTIR techniques. Raman spectrum of the ionic liquid is shown in Figure S3a. The peaks at 3447, 2972, and 1063 cm -1 can be assigned to O-H, C-H, and C-C stretching, respectively. The peaks at 1454 cm -1 can be indexed to CH2 and CH3 bending. The strong peak at 2200 cm -1 is due to the triple bond between C and N of anion molecules (cyanamide) of IL and the strong peaks at 181 and 76 cm -1 can be assigned to the lattice vibration. An FTIR spectrum of the ionic liquid is shown in Figure S3b. A broad peak at 3440 cm -1 can be assigned to OHstretching vibration. The peaks at 2967 and 2876 cm -1 can be referred to C-H stretching of CH2 and CH3 groups. The strong peaks at 2236 and 2131 cm -1 can be indexed to the triple bond between C and N while the peaks at 1643, 1467, and 1308 cm -1 relate to the pyrrolidinium cations of IL. Other peaks at around 500-1050 cm -1 are due to C-H bending and C-N bonding. To investigate phase deformation of Co(OH)2, the Co(OH)2 film after charged/discharged for 2000 cycles was measured using FTIR technique and compared with the as-prepared -Co(OH)2 film and IL as shown in Figure S3c. The FTIR peaks of the Co(OH)2 film after charged-discharged display the combination between -Co(OH)2 film and IL. 1 This reflects that IL can maintain the -phase of Co(OH)2 film after long cycling test.

Raman-FTIR spectra of the [BMpyr][DCA] ionic liquid
where is the measured current, is the scan rate, and are adjustable parameters. The b-value can be obtained from the slope of the linear curve between log( ) and log( ) when is the current, * is the surface concentration of the electrode material, is the transfer coefficient, is the chemical diffusion coefficient, is the number of electrons involved in the electrode reaction, is the surface area of the electrode materials, is the Faraday constant, is the molar gas constant, is the temperature, and the function (bt) represents the normalized current for a totally irreversible system as indicated by the CV response.
when is the capacitance, is the scan rate and is the surface area of the electrode materials.
when and refer to the current contribution from surface capacitive effects and diffusioncontrolled intercalation process. and are adjustable parameter at each potential. 6

Tuac plot 7 ;
The optical band gap energy has been determined by using Tauc relation; where  is the adsorption coefficient, h is the photon energy, k is a constant value of material, and Eg is the band gap energy. The value of n can be applied with various values such as 1/ 2 or 2. The absorption spectrum of each Co( OH) 2 film was used to estimate the Eg of Co( OH) 2. The optical band gap of each Co(OH)2 film was estimated by using linear fitting to find the interception in X-axis. The curve shows the square root of photoelectron emission yield as a function of scan energy. The interception point of the background and the yield line is a photoemission threshold energy, socalled the work function or ionization potential. The work function value correlates with vacuum state (energy level = 0 eV), therefore, this value is normally negative and corresponds to valence band (VB) edge of materials.    Figure S8. Photographs of the disassembled Co(OH)2 electrodes after charged-discharged for 2000 cycles; photoelectrode side (left) and opposite electrode side (right).

Quantum Chemical Calculations
The dicyanamide anion (DCA) molecule, N(CN)2 -, has been theoretically studied by ab-initio calculation. The ground-state geometry optimizations were carried out using the MP2 method and the aug-cc-pVTZ basis set. The frequency calculations were also performed at the same level of theory to check the optimized minima of this molecule. All calculations were performed with Gaussian 09 package. The optimized structure of the N( CN) 2is shown in Figure S9. It has a V-shape molecular configuration with the C1-N1-C2 bond angle of 118. 4. The distances of the both N1-C1 and N1-C2 are equal to 1. 32 Å. The highest occupied molecular orbital ( HOMO) is found to be delocalized predominantly on all atoms of DCA with *-orbital ( Figure S10). The HOMO energy is calculated to be -4.83 eV.
The HOMO energy of other anions in ILs commonly used for battery applications was also calculated by using the same level of theory. 8 The results are shown in Table S3. It can be clearly seen that the HOMO energy of the dicyanamide is closest to the valence band of Co( OH) 2 ( -5. 75 eV) as compared to other anions.